The present invention relates generally to the field of remote monitoring of fluids and, in particular, to a method and apparatus for the remote, near-continuous determination of the bubble rise velocity, viscosity, and other composition-dependent physical properties of a fluid. These physical properties, when correlated on a composition-dependent phase diagram for the fluid, can be used to ascertain the composition of the fluid. This invention has particular relevance to the remote monitoring of radioactive waste glasses.
The conversion of high-level liquid wastes arising from the reprocessing of spent nuclear fuels into a solid form is necessary to prevent the leakage of stored radioactive materials into the soil and to prepare the long-lived radioactive wastes for permanent disposal in geologic formations. Glass has been the preferred waste form because of its relative chemical stability, its capacity for incorporating various elements and compounds, and the existence of extensive experience with its production. Vitrification facilities for the conversion of high-level liquid wastes, primarily into borosilicate glass, have been in operation for several years and additional ones are in an advanced state of construction in many countries.
The properties of a glass are determined primarily by its composition, operating variables during its production, and the cooling rate after the glass is poured into a mold or canister. The chemical stability of a glass containing radioactive ingredients, as evidenced by a low leaching rate for most elements, is the property of paramount concern. Because variations in the chemical composition of the glass product may cause unacceptable variation in the leaching rates of certain radionuclides, great care is taken during the design of vitrification facilities to allow for very precise compounding of the ingredients and for continuous monitoring of sensitive operating variables. The high quality control standards in the production of the glass are part of the overall qualification process, which also includes monitoring the performance of the canisters, the transportation casks, and the geologic host repository.
However, it is recognized that regulatory authorities will require confirmatory test data, including the composition of the glass, when the glass is shipped to a repository. At present, the only way in which such a requirement could be met is to periodically remove glass samples, which are then chemically analyzed in a remotely operated analytical facility. Obtaining such samples is somewhat cumbersome because of the complexity of the remotely operated sampling equipment. The facilities for the transport and handling of the highly radioactive samples are equally complex and may require frequent maintenance. It is also possible that, because of the delays in obtaining the results of sample analyses, some canisters may be filled with glass that is outside the established specifications and the disposal of such material could pose problems. Sampling can only be done at finite intervals and some uncertainty concerning the quality of the glass produced between samplings would always remain.
Great strides have been made in recent years in the development of on-line control methods that are valuable not only in controlling various processes, but also in providing for the continuous monitoring of the quality of the product. The extension of such control methods to the production of radioactive waste glasses requires the availability of instruments that continuously provide an analysis of the glass product about to be poured into the canister. This is a formidable problem, considering the very high temperature of the molten glass, the intense radioactivity, and the complex composition of the glasses.
Thus, there exists a need for a method and apparatus for the near-continuous indication of the composition range, as well as deviations from a target product, for fluids such as radioactive glasses. Because it is possible to relate two or more composition-dependent physical properties of a fluid with its composition, an estimate of the composition of a specific type of glass can be determined from a measurement of the physical properties of the glass.
Most glasses, including radioactive waste glass forms, can be treated as ternary compounds. Triangular phase diagrams are frequently used to represent ternary systems or systems which can be simplified to pseudo-ternary. For instance, properties of borosilicate glasses can be shown on a ternary diagram in which SiO.sub.2, Na.sub.2 O, and B.sub.2 O.sub.3 are the constituent components. The waste glass from the Savannah River Plant can also be represented on a triangular phase diagram on which PHA (hydrolysis product from the cesium removal process), glass frit (having a specific composition), and sludge (precipitates formed during the neutralization of the high-level liquid wastes) are the primary constituents. Similarly, the glasses developed by the West Valley Demonstration Project can be treated as a pseudo-ternary system, with zeolite, sludge, and glass formers as the constituents.
Variation of the fluid's physical properties with composition can be represented on a triangular diagram as constant value lines for a given temperature (isopleths). By determining two physical properties at the same temperature, one can derive the composition of the fluid by locating the point where the two isopleths intersect. The location of the point of intersection is facilitated if the respective lines representing the two properties do not intersect at small angles.
A literature search of physical properties for borosilicate glasses reveals that melt viscosity varies appreciably with composition, the density changes only slightly, and the electrical resistivity is quite composition-dependent, but to a lesser extent than viscosity. Also, for this type of system, isopleth curves for viscosity/density and viscosity/electrical resistivity intersect at appreciable angles and thus, are appropriate variables for determining the composition. The density/electrical resistivity combination intersects at small angles and would consequently produce larger uncertainties in the derived composition values.
However, the selection of the physical properties used to determine the composition of the fluid must take into consideration the availability of suitable measuring methods and apparatus. For instance, viscosity is monitored to determine chain length during the polymerization of monomers to form polymers, which frequently occurs at high temperatures. In the case of radioactive waste glasses, it must be possible to measure these properties at temperatures above 1000.degree. C., in an intense radiation field, and in contact with corrosive molten glass. In addition, the measuring techniques also have to be amenable to remote operational control at distances in excess of 15-20 feet.
While suitable methods are available for the determination of density and electrical resistivity under these conditions, no practical methods for the measurement of viscosity are known that could be adapted for use in these conditions. Although many methods are available for the measurement of the viscosity of liquids, the high temperatures at which measurements must be carried out with molten glass and the corrosiveness of the liquid have restricted the choices to the falling ball, flow rate through an orifice, and the rotating spindle methods. The latter method, generally consisting of a Brookfield type instrument adapted for high-temperature use, is the technique favored for the laboratory determination of the viscosity of molten glass samples. However, a Brookfield type viscometer cannot be adapted for the continuous monitoring of glass melts, especially for remote operation in an intense radioactive field. Other methods, such as the falling ball method or the measurement of flow rate through an orifice, would also be difficult to develop for fluids such as glass melts. Measurements requiring rotating instrumentation controlled electrically would be subject to degradation in the high radioactive field present in waste glass processing. Furthermore, methods requiring rotation of the fluid are less accurate for non-Newtonian fluids and falling ball methods require the retrieval of the ball. Thus, a novel approach to continuous or near-continuous remote measurement of the viscosity or other physical properties of a fluid is needed.